This invention relates to a method for producing sodium sulfite (Na.sub.2 SO.sub.3) and/or bisulfite (NaHSO.sub.3) solutions (hereinafter sodium (bi)sulfite, collectively) in a packed tower apparatus.
Sodium sulfite and bisulfite can be produced from the reaction of sodium carbonate (soda ash) and sulfur dioxide in accordance with the following reactions: EQU Na.sub.2 CO.sub.3 +SO.sub.2 .fwdarw.Na.sub.2 SO.sub.3 +CO.sub.2 EQU Na.sub.2 CO.sub.3 +2SO.sub.2 +H.sub.2 O.fwdarw.2NaHSO.sub.3 +CO.sub.2
These reactions have been carried out in a number of ways, including countercurrent passage of sodium carbonate/sodium sulfite solution and SO.sub.2 gas through a series of absorber vessels, and the processes described in U.S. Pat. Nos. 2,245,697 to Melendy, 3,860,695 to Metzger et al. and 3,995,015 to Bean, which patents are incorporated herein by reference.
None of these patents disclose or discuss the use of a packed tower reactor for the manufacture of sodium sulfite and/or bisulfite. The advantages of packed tower reactors include low capital, simple equipment, and high throughput rates. On the other hand, control of this sort of system must be precise all of the time it is operated. Because there is relatively little material in process there is little capacity lag so that process upsets can develop rapidly.
In general, packed tower reactors are operated with countercurrent flow (i.e., in the same manner as some of the prior methods for making sodium (bi)sulfite). In the manufacture of sodium (bi)sulfite from soda ash and sulfur dioxide a stable foam is generated. This stable, viscous foam will not flow down against the rising stream of gas in the absorption column unless the gas velocity is quite low, generally less than one foot per second. Thus, it has been discovered that countercurrent operation in a packed tower is not feasible for use in the manufacture of sodium (bi)sulfites from sodium carbonate and SO.sub.2. In accordance with the invention, this problem is avoided by the use of a cocurrent flow arrangement. Cocurrent operation in a packed tower is not only feasible, it provides for surprisingly efficient operation. This invention thus provides an improved method for manufacturing (bi)sulfites using a packed column reactor with cocurrent flow.